Prior art includes the Prismatic Solar Beam Concentrator, U.S. Pat. No. 4,567,879, 2/4/86, by this inventor. This concentrator features two prisms having the same shape and outside dimensions as the inset solar prism but having a solid interior. The inset solar prism has structural cavities which reduce the volume and weight of the prism by 75 percent. The inset solar prism is designed to replace the solid prisms of the prismatic solar beam concentrator. The weight reduction is critical, because the prismatic solar beam concentrator will be mounted on the roofs of buildings.